1. Field of the Invention
The present invention relates to an image forming apparatus and more particularly to an image forming apparatus having a plurality of control modes for controlling a temperature of a thermal fixing apparatus and automatically selecting a corresponding control mode in accordance with a frequency of an image forming operation.
2. Description of the Related Art
In the conventional PPF (Plain Paper Facsimile), there has been known an apparatus for reducing the rate of operation of a heater for a thermal fixing apparatus so as to save power consumption and controlling a temperature of the thermal fixing apparatus to be lower than a printable temperature as the so-called sleep mode if time for which the facsimile does not receive a signal from an external unit continues for a predetermined period of time, that is, time when no printing operation for received data is performed continues for a predetermined period of time.
On the other hand, there has been known an apparatus in which the current supply to the heater for the thermal fixing apparatus is stopped, so that consumption power is largely saved and a current supply return operation is performed by inputting a receiving signal if the state that no signal is sent from the external unit continues for a predetermined period of time.
However, in the apparatus controlling the heater for the thermal fixing apparatus in the above-mentioned sleep mode and reducing the rate of operation of the heater for the thermal fixing apparatus, at night when the frequency of receiving is extremely low, current supply to the heater for the thermal fixing apparatus itself is that electrical power is wastefully consumed. Even if the heater is controlled in the sleep mode, this does not change the fact that current supply is always performed. Due to this, the lifetime of the thermal relating members such as a thermal fixing roll and a heater is shortened.
Moreover, in the control of stopping the current supply to the heater for the fixing device when no signal is supplied for a predetermined time, there are problems in that it takes a long time till the thermal fixing roll is stabilized and the fixing operation can be performed, that is, rise time (warm-up time), and a printing process cannot be performed at the same time with receiving the signal. In order to solve these problems, there can be used a process in which a memory may be provided in the facsimile and received data is once stored in the memory, and data is read out from the memory and a printing operation is performed when the temperature of the fixing device reaches to the temperature at which the fixing device can be fixed. However, in this case, the memory having extremely large capacities is needed in the facsimile apparatus, so that the manufacturing cost of the apparatus is increased.
In order to prevent the increase in the manufacturing cost of the apparatus, there is an apparatus in which a transmitting signal from an opponent is waited till receiving data becomes printable without providing the memory. However, the facsimile cannot be effectively used.